quote from the Katthavathuppakarana-Atthakatha (by Buddhoghosa) (p3 of Points of controversy, PTS) :

It talks about after the second council (about 100 years after Buddha parinibbana)

"Ten thousand of the of the Vajjiputtaka bhikkhus[after splitting from the good monks] seeking adherents among themselves, formed a school called the Mahasanghika [these then split several times] Thus from the school of the Mahasanghikas, in the second century only two schools seceded from the Theravada[note that the rightful monks are called Theravada by Buddhaghosa]-Mahimsinsasakas and Vajjiputtakas... [it lists more that split later]..Thus from the Theravada arose these eleven seceding bodies making 12 in all. And these 12 together the six schools of the Mahasanghikas constitute the 18 schools which arose in the second century. Of the eighteen, 17 are to be understood as schismatics,[b] the Theravadan only being non- schismatic[/b

]."""

The Theravada order has always being conservative. It is because of their great courage and wisdom that the true Dhamma is still here.

Take the case of the Abhayagiri sect which later arose as a distinct sect from the Mahavihara monks(who kept the Dhamma and Vinaya properly) which was mentioned by someone earlier in this thread

The Mahavamsa notes (p267 -268)p264 that a King helped to purify the sasana by suppression of a heresy. He seized bhikkhus dwelling in the Abhayagiri.."who had turned to the Vetulya [they followed some of the mahayana beliefs] doctrine and were like a thorn in the doctrine of the Buddha and he excommunicated them." It then notes that the thera sanghamitta (from south India was embittered against the goo bhikkhus of the Mahavihara and bided his time until the good king died and the next one Jetthatissa died. Then his time was ripe when the younger brother of Jetthatissa (mahasena ) came to power that the Thera sanghamitta , who dwelt at the Abhayagiri told the king that the Mahavihara teach a wrong doctrine and so the King made a proclamation telling the population that they could not feed any monk from the Mahavihara. "The good monks thus abandoned it.""

These good monks underwent incredible hardship as they people were forbidden to feed them. BUT they were steadfast in refusing to conduct acts of sangha such as reciting the Patimokkha with the Abhayagiri. Eventually things changed and the good monks were again allowed to be fed. I find this example of the conservatism of the ancient Theravada heroic and inspiring.

In fact the ancient monks would stand firm no matter how unpopular their stand was. To say that Theravada is merely some later sect who made base and false claims to follow the original teachings is extraordinarily ungrateful and foolish IMO.